In digital satellite receiving system, a control signal from a signal line between a set-top-box (STB) and a satellite receiver is transmitted to a Low Noise Block (LNB), a polar axis seat, and other devices to switch satellites. Satellite antenna and LNB device are active devices and they work outside, so they are easily effected by the environment. The STB transmits the control signal to a motor to rotate the polar axis seat for receiving satellite signals. The motor only works at the time while repositioning to focus on a satellite, so most of the time the motor is idle.